Polymers are cross-linked in order to alter their properties, for instance to impart improvements in resistance to creep, environmental stress crack (ESCR) and abrasion, to improve dimensional stability at elevated temperature, and the like. As an example, it is known to rotationally mould compositions of polyethylene and organic peroxides, utilizing the flow characteristics of the polyethylene before cross-linking during the rotational moulding step and then cross-linking the polyethylene to provide an article that has acceptable end-use properties. Another example is the addition of cross-linking agents to polymers used in the manufacture of wire and cable to improve heat distortion temperature properties.
While most cross-linking agents tend to be organic peroxides, other cross-linking agents are known. For instance, it is known to graft vinyl silanes e.g. vinyl trimethoxysilane (VTMS) and vinyl triethoxysilane (VTES), onto polymers, form the polymers into a desired shape and then subject the resultant article to moisture, whereupon the silane residue forms cross-links between polymer chains. The grafting reaction of the vinyl silane onto the polymer is usually carried out in the presence of an organic peroxide, and the cross-linking reaction with moisture is usually carried out in the presence of a cross-linking catalyst.
In order to utilize the moisture cross-linking capability of VTMS (or related compounds) in the manufacture of cross-linked polymer articles, the VTMS may be directly copolymerized with ethylene in a high pressure polymerization process; an example of the resultant polymer is SI-LINK.TM. polymer obtainable from Union Carbide Corporation. In such polymers, the VTMS is incorporated into the main polymer chain, and the polymers are generally considered to be cross-linkable analogues of low density polyethylene that is made in similar high pressure processes.
If it is desired to have another type of cross-linkable polymer e.g. a high or medium density polyethylene or an ethylene copolymer, including a linear low density polyethylene, or where it is not convenient or practical for other reasons to obtain a direct copolymer, the silane must be grafted onto the polymer in melt processing apparatus. Articles may then be formed directly from the grafted polymer, or the grafted polymer may be subsequently processed into articles. In either method, it is necessary to contact VTMS with the polymer in the presence of a grafting catalyst in melt processing apparatus e.g. an extruder. If the grafted polymer is to be used in a subsequent process i.e. not formed directly into an article, the molten grafted composition passing from the melt processing apparatus must be collected in a moistureproof container e.g. bags or bulk storage device; contact with moisture results in crosslinking of the polymer, prior to formation of an article.
The melt processing characteristics of the grafted polymer and the extent of undesired premature cross-linking in the extruder are extremely sensitive to traces of moisture; it has been suggested that levels of moisture of less than 200 ppm be present in catalyst and pigments. The container used to store the grafted composition must have very good moisture barrier properties, and such containers tend to be expensive. Premature cross-linking (scorch) results from silane, catalyst and moisture being in contact at high temperature and causes pits and pimples in the product. Moreover, each different grade of grafted polymer would need to be prepared and stored separately by the end-user. In addition to the need to store the grafted composition in a moisture-free environment, the grafted polymer tends to have a limited storage life e.g. six months. Thus, compositions that have been grafted prior to manufacture of articles therefrom are susceptible to deterioration in melt flow and other desired properties.
In forming operations, it is often possible to carry out the grafting reaction in the extruder of the forming apparatus, prior to the forming of the polymer into a shaped article. However, the need to handle one or more liquid feed streams e.g. VTMS, grafting catalyst and cross-linking catalyst, adds both cost and complexity to the forming operation. Liquid storage facilities and pumping, metering and monitoring systems are required, as it is important to maintain a constant ratio of polymer to VTMS and other liquids, and to obtain a uniform dispersion of the VTMS and other liquids in the polymer. This is particularly expensive if cross-linkable product is only required from time to time; at other times the systems could be idle. Costs are also incurred because additional attention by process operators is required. Another disadvantage is that, since the grafting process involves mixing molten polymer and liquid, special extruder screw designs that are not otherwise essential to the forming operation may be required.
Masterbatch compositions comprising a substrate or carrier and a silane having a hydrolysable radical and a functional group that can undergo chemical reactions with amino, carboxyl and/or amide groups are disclosed in UK 2 187 464A of E. Schmid, published Sep. 9, 1987. It is disclosed that the carrier or substrate used, when is concentrate is formed by absorption, is a block copolymer with hard block and an elastomeric segment. The examples show that ethylene/vinyl acetate copolymers absorbed 10-20% by weight of silane.